package calendar

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"sort"
	"strings"
	"time"
	_ "time/tzdata"

	ics "github.com/arran4/golang-ical"
	"github.com/golang/glog"
	rrule "github.com/teambition/rrule-go"
)

const (
	// EventsURL is the calendar from which we load public Hackerspace events.
	EventsURL = "https://owncloud.hackerspace.pl/remote.php/dav/public-calendars/g8toktZrA9fyAHNi/?export"
)

// eventsBySooner sorts upcoming events so the one that happens the soonest
// will be first in the list.
type eventBySooner []*UpcomingEvent

func (e eventBySooner) Len() int      { return len(e) }
func (e eventBySooner) Swap(i, j int) { e[i], e[j] = e[j], e[i] }
func (e eventBySooner) Less(i, j int) bool {
	a, b := e[i], e[j]
	if a.Start.Time == b.Start.Time {
		if a.End.Time == b.End.Time {
			return a.UID < b.UID
		}
		return a.End.Time.Before(b.End.Time)
	}
	return a.Start.Time.Before(b.Start.Time)
}

// parseUpcomingEvents generates a list of upcoming events from an open ICS/iCal file.
func parseUpcomingEvents(now time.Time, data io.Reader) ([]*UpcomingEvent, error) {
	cal, err := ics.ParseCalendar(data)
	if err != nil {
		return nil, fmt.Errorf("ParseCalendar(%q): %w", err)
	}

	var out []*UpcomingEvent
	for _, event := range cal.Events() {
		uidProp := event.GetProperty(ics.ComponentPropertyUniqueId)
		if uidProp == nil || uidProp.Value == "" {
			glog.Errorf("Event with no UID, ignoring: %+v", event)
			continue
		}
		uid := uidProp.Value

		summaryProp := event.GetProperty(ics.ComponentPropertySummary)
		if summaryProp == nil || summaryProp.Value == "" {
			glog.Errorf("Event %s has no summary, ignoring", uid)
		}
		summary := summaryProp.Value

		var description string
		descriptionProp := event.GetProperty(ics.ComponentPropertyDescription)
		if descriptionProp != nil && descriptionProp.Value != "" {
			// The ICS/iCal description has escaped newlines. Undo that.
			description = strings.ReplaceAll(descriptionProp.Value, `\n`, "\n")
		}

		status := event.GetProperty(ics.ComponentPropertyStatus)
		tentative := false
		if status != nil {
			if status.Value == string(ics.ObjectStatusCancelled) {
				// NextCloud only has CONFIRMED, CANCELELD and TENTATIVE for
				// events. We drop everything CANCELELD and keep things that are
				// TENTATIVE.
				continue
			}
			if status.Value == string(ics.ObjectStatusTentative) {
				tentative = true
			}
		}

		start, err := parseICSTime(event.GetProperty(ics.ComponentPropertyDtStart))
		if err != nil {
			glog.Errorf("Event %s has unparseable DTSTART, ignoring: %v", uid, err)
			continue
		}
		end, err := parseICSTime(event.GetProperty(ics.ComponentPropertyDtEnd))
		if err != nil {
			glog.Errorf("Event %s has unparseable DTEND, ignoring: %v", uid, err)
			continue
		}

		if (start.WholeDay && !end.WholeDay) || (!start.WholeDay && end.WholeDay) {
			glog.Errorf("Event %s has whole-day inconsistencies, start: %s, end: %s, ignoring", uid, start, end)
		}

		rruleS := event.GetProperty(ics.ComponentPropertyRrule)
		if rruleS != nil {
			rrule, err := rrule.StrToRRule(rruleS.Value)
			if err != nil {
				glog.Errorf("Event %s has unparseable RRULE, ignoring: %v", uid, err)
				continue
			}
			rrule.DTStart(start.Time)

			duration := end.Time.Sub(start.Time)
			if start.WholeDay {
				duration = time.Hour * 24
			}

			next := rrule.After(now, true)
			if next.IsZero() {
				continue
			}
			u := &UpcomingEvent{
				UID:         uid,
				Summary:     summary,
				Description: description,
				Start: &EventTime{
					Time:     next,
					WholeDay: start.WholeDay,
				},
				End: &EventTime{
					Time:     next.Add(duration),
					WholeDay: start.WholeDay,
				},
				Tentative: tentative,
			}
			out = append(out, u)
			continue
		}

		u := &UpcomingEvent{
			UID:         uid,
			Summary:     summary,
			Description: description,
			Start:       start,
			End:         end,
			Tentative:   tentative,
		}
		if u.Elapsed(now) {
			continue
		}

		out = append(out, u)
	}
	sort.Sort(eventBySooner(out))
	return out, nil
}

// GetUpcomingEvents returns all public Hackerspace events that are upcoming
// relative to the given time 'now' as per the Warsaw Hackerspace public
// calender (from owncloud.hackerspace.pl).
func GetUpcomingEvents(ctx context.Context, now time.Time) ([]*UpcomingEvent, error) {
	r, err := http.NewRequestWithContext(ctx, "GET", EventsURL, nil)
	if err != nil {
		return nil, fmt.Errorf("NewRequest(%q): %w", EventsURL, err)
	}
	res, err := http.DefaultClient.Do(r)
	if err != nil {
		return nil, fmt.Errorf("Do(%q): %w", EventsURL, err)
	}
	defer res.Body.Close()
	return parseUpcomingEvents(now, res.Body)
}